SiC (silicon carbide) is a wide bandgap semiconductor having a wide bandgap of 2.2 to 3.3 eV. Due to its excellent physical and chemical properties, it has been a focus of R&D as a device material since the 1960s. In particular, in recent years, SiC has been looked at closely as a material for blue to ultraviolet short wavelength light devices, high frequency electronic devices, and high-voltage and high-power electronic devices. In this regard, SiC is considered difficult to manufacture as a high quality large-sized single crystal. This has been one of the major factors obstructing the commercial application of SiC devices.
In the past, on a laboratory scale, for example, an SiC single crystal of a size enabling the fabrication of semiconductor devices had been obtained by the sublimation-recrystallization method (Rayleigh method). However, with this method, the obtained single crystal is small in area. Control of the dimensions, shape, and polytype and of the impurity carrier concentration is also not easy. On the other hand, chemical vapor deposition (CVD) is also being used to cause heteroepitaxial growth on silicon (Si) and other different substrates so as to grow cubic SiC single crystals. With this method, a large area single crystal is obtained, but there is also an approximately 20% lattice mismatch of SiC and Si etc., so it is only possible to grow an SiC single crystal with a large number of defects (up to 107/cm2). A high quality SiC single crystal could not be obtained.
Therefore, to solve these problems, the sublimation-recrystallization method using an SiC single crystal wafer as a seed crystal for sublimation-recrystallization has been proposed (see NPLT 1). If using this improved type sublimation-recrystallization method (modified Rayleigh method), it is possible to control the polytype of the SiC single crystal (6H type, 4H type, 15R type, etc.), shape, carrier type and concentration while growing the SiC single crystal. In addition, SiC comes in 200 or more polytypes, but from the viewpoint of the productivity of the crystal and the performance of the electronic device, 4H polytype is considered the most superior. Most commercially produced SiC single crystals are 4H type. Further, regarding the conductivity, as the dopant, nitrogen is easy to handle. On this point, single crystal ingots are almost always grown by n-type conductivity. However, in applications for communication devices, crystals which contain almost no dopant elements and are high in resistivity are also being manufactured.
At the present time, diameter of 51 mm (2 inch) to 100 mm SiC single crystal wafers are being cut out from SiC single crystals fabricated by the sublimation-recrystallization method and are being used for fabrication of devices in the electrical power and electronics fields etc. Furthermore, 150 mm wafers are also starting to be marketed (see NPLT 2). Full scale commercialization of devices using 100 mm or 150 mm wafers is anticipated. In view of this situation, the technologies for improving the productivity of SiC ingots and the yield of crystal growth and thus leading to a drop in costs have become to be increasingly important.
The main method for manufacturing a SiC single crystal ingot is, as explained above, the modified Rayleigh method. Solution growth (see NPLT 3), high temperature CVD (see NPLT 4), etc. are also being performed at the research level, but these do not approach the modified Rayleigh method in terms of productivity (number of wafers able to be obtained per ingot or success rate of growth of high quality ingots) and quality. However, the modified Rayleigh method is a process performed at a ultra-high temperature of 2000° C. or more. Further, since the material is supplied in a vapor phase on growing surface, etc., control of the growth conditions is technically difficult. The precise figures of the different SiC wafer manufacturers have not been disclosed publicly, but the number of wafers able to be obtained per ingot and the success rate of growth of high quality ingots reportedly do not approach those of the Si and other highly technically advanced industries. From the viewpoint of pursuit of commercial profit, further improvement is sought in the productivity of SiC single crystals.
For the above purpose, conditions for manufacturing a SiC single crystal ingot by the sublimation-recrystallization method are the focus of intense R&D. These mostly relate to the material and structure of the crucible forming the growth vessel, the purity and particle size of the starting material, and the ingredients and type of the atmospheric gas, but the temperature of the starting material or growth surface can be said to be the most important factor in the success rate or the yield of the crystal growth. The reason why is that, in growth of a single crystal, not limited to SiC, the temperature directly impacts the conditions of both the sublimation or melting of the starting material and the solidification or recrystallization of the single crystal. In SiC single crystal growth as well, only naturally, an emission pyrometer or other general method was used to measure the temperature of the crucible.
PLT 1 discloses a silicon carbide single crystal ingot with a seed crystal obtained by growing a silicon carbide single crystal by a method for manufacturing a silicon carbide single crystal including a step of using the sublimation-recrystallization method using a seed crystal for silicon carbide single crystal growth having a groove of a width of 0.7 mm to less than 2 mm at its growth face so as to grow a silicon carbide single crystal on the seed crystal. The seed crystal is placed inside the crucible together with the SiC starting material powder and is heated in an argon or other inert gas atmosphere to 2000 to 2400° C. The temperature gradient is set so that the seed crystal becomes somewhat lower in temperature compared with the starting material powder.
However, SiC single crystal growth is ultra-high temperature (2000° C. to 2600° C., see NPLT 4) and vapor phase growth process, so the growth crucible is semisealed in structure. The temperatures of the starting material and grown crystal are difficult to directly measure.
Further, in the high temperature SiC sublimation-recrystallization method, the general practice is to use an induction heating furnace surrounding the crucible with an induction coil. Due to the characteristics of induction heating, the position where the sublimation reaction occurs is near the side walls of the crucible in which the starting material is held, so direct measurement of the starting material temperature is not possible. For this reason, the temperature measurement point used for estimating the temperature has to be made the bottom surface of the crucible far from the position where the sublimation reaction occurs. In PLT 1 as well, the starting material temperature is estimated from the temperature of the bottom surface of the crucible.